Well, actually an Agricultural civ wants Rivers right away, moreso than other civs. With Iroquois, especially, beelining for Monarchy is a strong option as the same tech line also gives you Mounted Warriors. Moreover, Monarchy is much more manageable for large population civs because you can use as much as 3 units as military police. It helps to ameliorate the happiness issues you are likely to face before cathedrals become available. Republic may be better if you have plenty of luxuries or the Oracle.